Touch panels have recently become widely adopted as the input device for high-end portable electronic products such as smart-phones and tablet devices. Although, a number of different technologies can be used to create these touch panels, capacitive systems have proven to be the most popular due to their accuracy, durability and ability to detect touch input events with little or no activation force.
The most basic method of capacitive sensing for touch panels is the surface capacitive method (also known as self-capacitance), for example as disclosed in U.S. Pat. No. 4,293,734 (Pepper, Oct. 6, 1981). A typical implementation of a surface capacitance type touch panel is illustrated in FIG. 1 and comprises a transparent substrate 10, the surface of which is coated with a conductive material that forms a sensing electrode 11. One or more voltage sources 12 are connected to the sensing electrode, for example at each corner, and are used to generate an electrostatic field above the substrate. When a conductive object, such as a human finger 13, comes into close proximity to the sensing electrode, a capacitor 14 is dynamically formed between the sensing electrode 11 and the finger 13 and this field is disturbed. The capacitor 14 causes a change in the amount of current drawn from the voltage sources 12 wherein the magnitude of current change is related to the distance between the finger location and the point at which the voltage source is connected to the sensing electrode. Current sensors 15 are provided to measure the current drawn from each voltage source 12 and the location of the touch input event is calculated by comparing the magnitude of the current measured at each source. Although simple in construction and operation, surface capacitive type touch panels are unable to detect multiple simultaneous touch input events as occurs when, for example, two or more fingers are in contact with the touch panel.
Another well-known method of capacitive sensing applied to touch panels is the projected capacitive method (also known as mutual capacitance). In this method, as shown in FIG. 2, a drive electrode 20 and sense electrode 21 are formed on a transparent substrate (not shown). The drive electrode 20 is fed with a changing voltage or excitation signal by a voltage source 22. A signal is then induced on the adjacent sense electrode 21 by means of capacitive coupling via the mutual coupling capacitor 23 formed between the drive electrode 20 and sense electrode 21. A current measurement means 24 is connected to the sense electrode 21 and provides a measurement of the size of the mutual coupling capacitor 23. When a conductive object such as a finger 13 is brought to close proximity to both electrodes, it forms a first dynamic capacitor to the drive electrode 27 and a second dynamic capacitor to the sense electrode 28. The effect of these dynamically formed capacitances is manifested as a reduction of the amount of capacitive coupling in between the drive and sense electrodes and hence a reduction in the magnitude of the signal measured by the current measurement means 24 attached to the sense electrode 21. As is well-known and disclosed, for example in U.S. Pat. No. 7,663,607 (Hotelling, Feb. 6, 2010), by arranging a plurality of drive and sense electrodes in a grid, this projected capacitance sensing method may be used to form a touch panel device. An advantage of the projected capacitance sensing method over the surface capacitance method is that multiple simultaneous touch input events may be detected.
If the sensitivity of the projected capacitive touch sensor is sufficiently high, objects may be detected at some distance from the sensor substrate. In principle, this capability can be used to add new functions to a user interface. For example, specific action may be implemented when an object (such as a stylus or a finger) is held just above a point on the touch sensor. For example, the finger may cause an item in the user interface over which the object is located to be pre-selected, highlighted or the like. For reliable operation, the touch sensor must calculate height accurately enough to distinguish between objects that touch the sensor substrate, and objects that are held in close proximity to the substrate. Equally, the touch sensor and controller circuitry may be designed to recognise complex gestures made by the user in the 3D space above the sensor substrate. This requires the touch sensor to accurately calculate the heights of multiple objects.
The simplest way of determining object height is to examine the magnitude of the change in capacitance detected by the touch panel. However, FIG. 3 illustrates two cases that may typically cause unreliable operation. For example, in a first case, a given capacitance change may be caused by a large object 320 at some distance from the touch panel 310 whilst in a second case a similar change in capacitance may be caused by a small object 330 at a second closer distance from the sensor. It is therefore not possible to determine object height by examining the magnitude of the capacitance change alone. Further, the change in capacitance will also be influenced by the conductivity of the object, and by the resistance of its path to ground, both of which are typically unknown.
Another way of determining object height is ‘triangulation’, which involves combining proximity readings from multiple positions on the touch panel 310 (more specifically the intersections of the drive electrodes and the sense electrodes). This principle is illustrated in FIG. 4. A distant object 410 will cause a similar change in capacitance to be measured at the different positions 420 and 430. Conversely, a close object 440 will cause a much greater change in capacitance to be measured at position 430, directly beneath the close object 440, than at position 420. By examining the normalised distribution of the measured capacitance change with distance across the sensor substrate, as shown in FIG. 5, the height of the object may be inferred independently of the absolute capacitance changes. The measure is therefore independent of the object's conductivity and resistance to ground. However, using this method it can still be difficult to separate the object's size from its height. This is because a large object in close proximity, and a small object at a distance, will each produce a similarly uniform distribution of capacitance change across the substrate.
Similarly, the calculation becomes complex when the position of multiple objects must be determined, at some distance above the sensor substrate. Computationally intensive methods are then required, such as those proposed by Van Berkel and Lionheart (“Reconstruction of a grounded object in an electrostatic halfspace with an indicator function”, Inverse Problems in Science and Engineering, Vol. 15, No. 6, September 2007).
Finally, if the object is small and in close proximity, its influence may become localised to the one intersection directly beneath it. This means that the height determination algorithm becomes ill-conditioned when determining whether actual contact has been made with the touch sensor substrate.
FIG. 6 shows another way of determining the height of an object 605, which involves comparing two projected capacitance measurements taken from approximately the same point, but using different electrode geometries. For example, U.S. Pat. No. 7,098,673 (Launay, Aug. 29, 2006) describes adding an ‘auxiliary measurement electrode’ 610 between the parallel drive electrode 620 and sense electrode 630 of a discrete capacitive sensor. However, this method may not be applied to projected capacitance sensor matrices such as those used in touch panels.
Accordingly, there exists no satisfactory means of reliably determining an object's height using a projected capacitance sensor. As described above known schemes are either ill-conditioned, computationally intensive, or do not apply to projected capacitance sensor matrices.